Updated

House Minority Leader Kevin McCarthy accused Democrats of "pushing conspiracy theories" in accusing President Trump of trying to undermine the U.S. Postal Service (USPS) ahead of the Nov. 3 presidential election, as House Speaker Nancy Pelosi calls members back from their August recess to work on legislation to address Trump's alleged threats against the institution.

McCarthy, R-Calif., leveled the allegation against Democrats in a tweet Monday afternoon after Pelosi, D-Calif., charged House Majority Leader Steny Hoyer, D-Md., with setting up a schedule for the legislation.

"Democrats have no shame," McCarthy said. "They are pushing conspiracy theories about the USPS to undermine faith in the election and distract from their own failures. Whether Americans choose to vote in person or vote absentee, I have full confidence in the integrity of our electoral process."

PELOSI CALLS MEMBERS BACK TO CAPITOL HILL TO CONSIDER USPS LEGISLATION, CALLS POST OFFICE 'ELECTION CENTRAL'

Pelosi Sunday said that she is "calling upon the House to return to session later this week to vote on Oversight and Reform Committee Chairwoman [Carolyn] Maloney’s 'Delivering for America Act,'" a bill that would mandate no cuts to USPS' level of operations compared to what it had at the beginning of the year.

The brewing legislative battle comes as Trump spars with Democrats over efforts in many states to implement universal mail-in voting – the practice of mailing actual mail-in ballots, rather than requests for ballots, to every registered voter. Not every state is setting up universal mail-in voting, and some states have used universal mail-in voting successfully in the past. But some on the right, including the president, have warned that quickly changing states' electoral systems and sending ballots to the addresses of all registered voters – who in many cases have moved or died – could make the election ripe for fraud.

Democrats have cited studies showing that there has been very little mail-in vote fraud in the past, while Republicans have pointed to examples of trouble with universal mail-in voting during the pandemic, including issues in elections in Paterson, N.J., and elsewhere.

Further unnerving Democrats have been actions by the newly-minted Postmaster General Louis Dejoy that critics say have slowed down mail delivery, ostensibly in an effort to save money for the perpetually-in-the-red Post Office, ahead of the November election.

Other comments Trump made in an interview on FOX Business last week – regarding funding provisions for the Post Office that Democrats asked for in negotiations over the latest coronavirus relief package – also alarmed left-leaning legislators.

"If we don’t make a deal, that means they don’t get the money," Trump said. "That means they can’t have universal mail-in voting; they just can’t have it."

DEMOCRATS DEMAND POSTMASTER GENERAL, USPS CHAIRMAN TO TESTIFY

Pelosi responded in a joint statement with Senate Minority Leader Chuch Schumer, D-N.Y., Rep. Maloney, D-N.Y., and Sen. Gary Peters D-Mich.: "The president has explicitly stated his intention to manipulate the Postal Service to deny eligible voters access to the ballot in pursuit of his own reelection. Alarmingly, the postmaster general – a Trump mega-donor – has acted as an accomplice in the president’s campaign to cheat in the election, as he launches sweeping new operational changes that degrade delivery standards and delay the mail."

The Post Office, of course, handles vast amounts of mail per day and would likely be able to handle an election-related surge of mail-in ballots without the specific funding provisions Democrats were asking for. And Trump, prone to bluster and off-the-cuff comments that land him in hot water, later walked back his position to an extent, saying that he does not oppose additional funding for USPS but rather has problems with Democrats' other demands related to the coronavirus legislation.

But USPS has warned 46 states and Washington, D.C., that it may not be able to get all mail-in ballots delivered in time to be counted according to those jurisdictions' election laws. In those letters, reported by the Washington Post, USPS recommends that states move up their deadlines for voters to send their mail-in ballots to ensure they are received by Election Day. And a July experiment conducted by CBS with mock ballots resulted in about 3% of those ballots not reaching their destination.

The conflicting statements and anecdotes, and the resulting confusion, are part of the reason why Pelosi and the other Democratic congressional headers are planning a hearing Monday, in which why hope to have DeJoy testify.

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"House Democrats ... are ramping up their ongoing investigation by requesting that Postmaster General Louis DeJoy and Chairman of the USPS Board of Governors Robert Duncan testify at an urgent hearing before the committee on Aug. 24," the Democrats' statement read. "The hearing will examine the sweeping operational and organizational changes at the Postal Service that experts warn could degrade delivery standards, slow the mail and potentially impair the rights of eligible Americans to cast their votes through the mail in the upcoming November elections."

The Republicans of the House Oversight Committee, however, responded in a Monday statement saying they aimed to "put this conspiracy theory to bed," seemingly meaning in next Monday's hearing.

"The Democrats’ wild and baseless conspiracy theory about the U.S. Postal Service is irresponsible and only undermines the American people’s faith in the integrity of the election and our institutions," House Oversight Committee Ranking Member James Comer, R-Ky., said. "The Committee on Oversight and Reform has a tremendous opportunity to put this conspiracy theory to bed and I hope my Democratic colleagues will join me in reviewing the facts and identifying areas of reform so that USPS works best for the American people."

Fox News' Morgan Phillips and The Associated Press contributed to this report.